Thermally crystallizable, or as otherwise stated, thermally devitrifiable glass compositions have heretofore attained a stature of substantial useful significance in numerous and varied areas of technology and commercial application. For example, such glasses are commonly utilized as a sealant or bonding medium for application to and with a wide variety of materials such as, among others, glasses, metals, ceramics, and the like, and in he fabrication of cathode-ray tubes, microcircuits, insulated wires, etc. In consequence of the estensive employment of these thermally devitrifiable, or thermally crystallizable, glasses for sealing and bonding purposes they are also referred to frequently as being sealing glasses, or solder glasses; and when employed for such purposes are ordinarily utilized in a finely comminuted form either with or without additional temporary binders or adhesives.
In a more definitive sense, however, these crystallizable glasses are of a character such that, as initially prepared they are in a non-crystalline state and possess many of the attributes and characteristics of vitreous glasses. However, unlike common or ordinary vitreous glasses, thermally crystallizable glasses, as they will hereinafter be referred to and such as are contemplated within the intendment of the present invention, also possess the unusual capability of being thermally converted into a generally monolithic, essentially crystalline body composed of about 90% or more of well-integrated crystallized glass. In the latter crystallized condition such glasses exhibit many advantageous characteristics which are not exhibited in the vitreous condition. For example, to name a few, such advantageous characteristics include greatly increased physical stength and durability, as well as, very importantly, greatly increased resistance to strength diminution during progressive elevation of temperature. Thus, briefly stated, glasses of the character herein contemplated are those which, by analogy, are unlike ordinary vitreous glasses in that they are capable of being thermally converted from a vitreous condition to a predominantly crystalline condition when exposed for a time interval of approximately 60 minutes duration to temperatures in the range of approximately 60.degree.C. above the fiber softening-point temperature of the vitreous glass.
Due to the wide variety of specific commercial applications utilizing thermally crystallizable glasses, the specific requisite properties of such glasses are often tailored, or modified, to meet specific needs of a given situation of use and to enhance the attainment of a specific desired result. For example, thermally crystallizable glasses are avilable having compositions individually tailored to provide fiber softening-point temperatures commensurate with the specific needs of the user. Also, the inclusion of various compositional constituents for the purposes, among others, of tailoring the thermal expansion and contraction properties, thermal conductivity and dielectric properties, and chemical durability of the resultant crystallized glass are extensively known in the art. For example, as indicated in U.S. Pat. No. 3,250,631, issued to Kenneth G. Lusher and assigned to the assignee of the present invention, inert refractory metal oxides may be included in a thermally crystallizable glass composition for the specific purpose of individually tailoring or modifying the thermal expansion characteristics thereof without appreciably affecting such other characteristics as the sealing temperature and flow characteristics of the glass. Similar practices of modifying or tailoring thermally crystallizable glass compositions are further evident in U.S. Pat. No. 3,291,586 wherein mixtures of finely divided thermally non-crystallizable glass and finely divided thermally crystallizable glass are employed together to restrict the extent of overall crystallization occurring during the thermal crystallization process. According to other known concepts copper oxide may be included to provide electrical conductivity or, as disclosed in U.S. Pat. No. 3,389,458, constituents such as TiO.sub.2 may be utilized to enhance the dielectric properties of the resultant thermally crystallized glass. Thus, while the broad concept of modifying or tailoring various individual characteristics of thermally crystallizable glasses is known, and while warp and means are known for modifying or tailoring certain properties and characteristics thereof; the problems of providing a thermally crystallizable glass possessing precisely modifiable and controllable rates of crystallization and flow has, so far as is known, remained as a problem which is generally common to most all thermally crystallizable glasses including such thermally crystallizable glasses as those mentioned above having other individually tailored or modified characteristics.
In keeping with the foregoing, it is important to bear in mind that, irrespective of the particular composition of the thermally crystallizable glass or the particular processing technique or procedure employed in thermally crystallizing the same, proper and efficient process control is ordinarily dependent upon the crystallization and flow rates or characteristics of the glass in order to ensure that the processing techniques for thermally crystallizing the glass, once having been properly developed and effectively established, may be regularly employed in the course of standardized production procedures to effect a continually reproducible, high quality product unaffected by variations in the crystallization and flow characteristics of the thermally crystallized glass.
Of no less importance is the time-temperature control factor which is commonly a governing factor in processing steps and operations employing thermally crystallizable glass compositions. For example, many processes wherein thermally crystallizable glass compositions are utilized are restricted to the employment of critically precise time-temperature limitations which if not maintained are productive of a non-acceptable product. To be suitable for use in such processes, the thermally glass composition must be capable of exhibiting precisely predictable thermal crystallization and flow rates compatible with the precise time-temperature limitations of such processes. Otherwise stated, many processes otherwise especially well-suited for and having need for the utilization of thermally crystallizable glass compositions have heretofore avoided the use of thermally crystallizable glass compositions because of processing limitations which are not subject to the extent of variation necessary to accommodate and off-set variations in the rates of crystallization and flow commonly occurring in thermally crystallizable glass compositions of even the most exacting uniformity heretofore otainable.
Accordingly, it is a principal objective of the present invention to provide a thermally crystallizable glass in a finely comminuted form in which it may be conveniently utilized in a wide variety of commercial applications and yet possess precisely predictable properties of crystallization and flow during the thermal crystallization thereof.
Another objective of the present invention is the provision of a thermally crystallizable glass which, in addition to satisfying the foregoing objectives is capable of use as a "master blend" to precisely alter the rates of crystallization and flow of other thermally crystallizable glasses of similar composition during thermal crystallization thereof.
According to another aspect of the present invention, it is an objective to provide a method of producing a thermally crystallizable glass composition capable of attaining the foregoing objectives.
Another objective, in keeping with this latter aspect of the present invention, is the provision of a method whereby large quantities of essentially uncrystallized particles of thermally crystallizable glass may be produced in uniform finely comminuted form and by the uniform dispersion therein of from 1 - 10 parts per million of crystallized glass be tailored to possess precise predeterminable rates of thermal crystallization and flow.
A further and more specific objective of the present invention is the provision of a method for providing a "master blend" of finely comminuted particles of thermally crystallizable and thermally crystallized glass for use in controlling the crystallization and flow characteristics of other thermally crystallizable glasses of similar composition and which is characterized by the steps of providing a quantity of uncrystallized chips of thermally crystallizable glass; providing a quantity of finely comminuted particles of thermally crystallized glass, admixing the crystallized glass particles with the uncrystallized glass chips in a ratio of between about 100 and 225 parts of crystallized glass particles to one million parts of uncrystallized glass chips; reducing the particle size of the admixed chips of uncrystallized glass and the particles of crystallized glass to a particle size wherein essentially all of the admixed particles are of -100 mesh screen size and such that 65-78 % by weight are of -325 mesh screen size; uniformly blending the admixed particles of crystallized and uncrystallized glass to thereby form a "master blend"; thereafter blending the master blend with a further quantity of finely comminuted, uncrystallized particles of thermally crystallizable glass in a ratio providing a uniform blend of between about 1 and 10, parts of fully crystallized glass particles per million parts of uncrystallized glass particles.
In accordance with one aspect of the invention, the foregoing objectives are attained by providing an exceedingly uniform blend composed of finely comminuted, uncrystallized particles of thermally crystallizable glass and finely comminuted particles of fully crystallized glass in a ratio such that the crystallized particles represent only between about 1 and 10 parts per million parts of the uncrystallized particles present in the resultant blend. The method of attaining a uniform blending of such a minute quantity of crystallized glass particles is exceedingly important and constitutes another aspect of the invention.
In carrying out the method aspect of the invention, it has been found that a very high degree of uniformity can be effectively accomplished by precisely controlling the particle size of both the crystallizable and the crystallized components of the glass and by combining such particle size control with a step-wise controlled blending.
In this latter respect, a quantity of thermally crystallizable glass is prepared in the form of thin uncrystallized chips having a thickness of about 20-25 mils. The chips are then admixed, comminuted and uniformly blended together with crystallized particles of thermally crystallizable glass. The comminution and blending is preferably accomplished by combining the chips of frangible uncrystallized glass with crystallized glass particles having a particle size ranging between about -20 and +80 U.S. Series Sieve screen size and in a ratio of between about 100 to 225 parts by weight of crystallized glass particles to one million parts by weight of uncrystallized glass chips. Thereafter the comminution and blending may be preferably carried out concurrently in a suitable mill, such as a pebble mill. The milling and blending of the crystallized glass particles and uncrystallized chips is continued sufficiently to form a uniform blend and particle size distribution in which essentially all of the particles will pass through a -100 U.S. Series Sieve screen and such that between about 65 and 78 percent by weight thereof will pass through a -325 U.S. Series Sieve screen. The milled blend which constitutes a master blend or control blend is then in suitable condition to be further blended with other large quantities of finely comminuted, uncrystallized particles of thermally crystallizable glass similar in character to the constituent oxide composition of the uncrystallized particle portion of the master blend and in a ratio to provide from about 1-10 parts by weight of crystallized glass particles to one million parts by weight of uncrystallized glass particles in the final blend or so-called "product blend."
Other objects, advantages and aspects of the present invention, together with the specific nature thereof, will become readily apparent to those ordinarily skilled in the art from the following detailed description, wherein by way of example only, several preferred embodiments of the invention are described in specific detail.